1. Field of the Invention
The present invention relates to a method of removing a material layer, and more particularly, to a method that removes a material layer by performing a wet etching process.
2. Description of the Prior Art
Transistors are important electrical components utilized in the manufacture of integrated circuits. As semiconductors become smaller and smaller, the manufacture of transistors has undergone great improvement in order to produce transistors of small size and high quality.
In current transistor manufacturing processes, silicide is widely used in the fabrication of metal-oxide-semiconductor (MOS) transistors on a wafer. For example, the silicide is often formed on the surface of a gate electrode to provide a good ohmic contact at the interface between the gate electrode and a subsequently formed metal layer. Because the materials of the contact plugs are conductive metal, such as tungsten (W), the polycrystalline silicon or monocrystalline silicon of the gate structure and the source/drain regions have a bad electrical conduction with the contact plugs. Therefore, the silicide is formed on the gate structure and the source/drain regions to improve the ohmic contact between the gate structure, the source/drain regions and the conduct plugs, and to enable the sheet resistances of the source/drain regions to decrease at the same time. Among silicide constituents, nickel silicon is considered important to the development of manufacturing processes in the 65 nm MOS technology because of the characteristics including low electrical resistance, low silicon consumption, good resistance behavior in narrow lines, and low processing temperature.
However, since the nickel silicide has low thermal stability, it is possible that nickel may penetrate through the interface between metal and silicon down to the gate electrode to cause spiking effect, or the nickel may laterally diffuse to the channel region to cause nickel-piping effect. In addition, an agglomerating of the nickel silicide might occur. The agglomeration of the silicide increases the contact resistance of the contact plug so that junction leakage occurs. To prevent these defects, a low concentration stable metal is added into the metal layer, which is utilized to form the silicide. For example, 3-8 weight percent (wt %) of platinum (Pt) is added in the nickel metal layer, which is utilized to form the silicide. Because platinum has a stable chemical property, platinum improves the thermal stability of nickel silicon. So, the nickel silicon will not agglomerate at a higher temperature after platinum is added.
Please refer to FIGS. 1-4. FIGS. 1-4 are schematic diagrams illustrating a method of manufacturing the silicide by utilizing a self-aligned silicidation (salicide) process in the prior art. As shown in FIG. 1, a semiconductor wafer 10 having a semiconductor substrate 12 is prepared. A gate structure 20 is first formed on the semiconductor substrate 12, and the gate structure 20 includes a gate insulating layer 14 and a gate conductive layer 16. Subsequently, an ion implantation is performed, and source/drain extensions 26 (also called lightly doped drains, LDD) are formed in the semiconductor substrate 12 of the two opposite sides of the gate structure 20. Next, a spacer 18 is formed around the gate structure 20. The gate structure 20 and the spacer 18 are utilized as implanting masks and an ion implantation is processed to form the source/drain regions 28 in the semiconductor substrate 12.
Thereafter, a thin film deposition process is performed so that a metal layer 22 is uniformly formed on the semiconductor substrate 12 and the gate 20. The metal layer 22 includes 3-8 wt % of platinum and 92-97 wt % of nickel. Afterward, a TiN layer 24 is sputtered on the metal layer 22.
As shown in FIG. 2, a first rapid thermal process (RTP) is performed so that partial metal layer 22 reacts with the silicon in the gate conductive layer 16 positioned under the metal layer 22 and in the source/drain regions 28 to form a transitional silicide 30, such as Ni2Si.
Next, as shown in FIG. 3, a sulfuric acid-hydrogen peroxide mixture (SPM) etching process is performed in an etching equipment 100 so that the TiN layer 24 and the un-reacted nickel in the metal layer 22 are removed. Generally speaking, the etching equipment 100 includes a rotating platform 110, and the semiconductor wafer 10 is fixed on the rotating platform 110. A center pipe 112 positioned on the center region of the rotating platform 110 is included in the etching equipment 100. The center pipe 112 has a plurality of nozzles 122, and the etching solution 130 of the SPM etching process is sprayed from the nozzles 122.
Furthermore, as shown in FIG. 4, a hydrochloric acid hydrogen peroxide mixture (HPM) etching process can be performed in the etching equipment 100. The HPM reacts with un-reacted platinum positioned above the transitional silicide 30 to generate the soluble complex ions. Thus, the un-reacted platinum is stripped. The second RTP is next performed so as to turn the transitional silicide 30 into a silicide 32, which has a lower resistance, such as NiSi.
Although platinum is a noble metal element with stable chemistry properties, and is helpful to improve the thermal stability of nickel silicide, platinum also has the property of being difficult to etch. As a result, platinum remains after removing the un-reacted metal layer. In order to strip the platinum residue effectively, it requires an extremely long etching time. Accordingly, it not only wastes money, but also damages the transitional silicide. The above-mentioned etching solutions include hydrogen peroxide, hydrochloric acid, and chlorine (Cl2). These materials damage the transitional silicide 30. Parts of the transitional silicide are even stripped by these etching solutions. In order to shorten the etching time, an HPM solution having high concentration and high temperature should be used in the etching process. However, it results in an awful etching uniformity of the semiconductor wafer 10, and the product yield is decreased considerably.